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Is this right?

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deltarider:
Just read this in the international forum.
Question was: I just wanted to verify that turning IN (clockwise) the airscrews makes it run leaner. I ask only because I've been working under the assumption that turning them out brings in more air thus more lean. This is for a 1976 750F with 064A carbs.

The answer given is:
Yes.
The idle-air screw in those carbs (all of the 750 roundtop carbs) controls the amount of fuel-air mixture that gets pulled up for each intake stroke. It is already mixed as fuel & air inside the tiny tube on the top of the idle jet, where those holes are, when air from the little brass orifice at the back of the carb's bell met the rising fuel in the jet's metering hole. Then, the position of the screw admits more (or less) of this mixture during the intake suction.

I think the internet's description of these as "air screws" or "idle fuel screws" may be adding much to the confusion about these gizmos. It is the size of the jet (#40) that determines the RATIO of air-to-fuel, and the screw then controls HOW MUCH of this mixture gets added for each stroke. If you add too much, it runs rich, and vice-versa.

When the screw is turned more inward, it increases the amount of time required for the fuel-air mixture to rise up into the carb's throat. If turned more outward it lets this happen sooner during the intake stroke. In between each intake stroke the fuel falls back down to the little emulsifier holes in the top of those idle jets, and if the idle adjuster screw is turned inward more, it pinches off the flow, making things happen slower. This in turn makes the amount of [fuel + air] entering the engine less for that stroke, so ti runs leaner.

Years ago (notably in Honda's Shop Manuals for twins) Honda used the ambiguous translation phrase for the use of this screw that said, "For richness, turn screw in". It was this poor translation that seems to have started the confusion: in English this means, "If the engine is running rich, turn the screw inward". That adjustment reduces the amount of fuel/air mix being admitted, making it run leaner. This was an issue with the Honda Twins, as the vibration often made the screws slowly back out, causing richness and fouled sparkplugs that brought the bikes in for a tuneup. The CB350 twin was notorious for this.

How about the CB500/550s with the oldstyle carbs: 627B, 649A, 069A and 022A?

K2-K6:
The twins in majority use CV type carbs (can't recall throttle slide example) that being the case, they operate differently and may cause confusion in using as a reference to understand the four cylinder engine and carburettor setting.

Johnwebley:
The smaller twins CB200 era use slides, and operate the same as the 500,



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Herr Supersport:
In general I believe every of his words.
But never understood his explanations to the airscrews.

Just to mention: some years ago I tested my carbs [CB400f] with colortunes. While idling turning the screw inwards changed the colour to orange/rich, turning out changed to bright-blue/lean.

Possibly an answer to 'is that right?'

deltarider:
Long time ago I have used a Colortune. When I finally got the blue color, bike wouldn't accelerate anymore. ;D
I know from experience which way airscrews should be turned. I don't understand Hondaman's view on the matter.
K2-K6 once gave here an on insight how the airscrews function and I found his explanation much better.

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